Check the period
Two lines can share a chart without sharing a fair comparison. The start, end and spacing of the observations all matter.
Make sure the observations cover the same stretch of time.
Start with the idea
A period is the interval covered by the chart. A frequency is how often observations appear within that interval. A monthly series and a daily series may cover the same year, but they reveal different detail. A monthly chart will not show every daily movement.
Check that both series begin and end on matching dates. Comparing a three-month change with a twelve-month change does not answer which series changed more over the same period. Do not assume that four evenly spaced marks mean four days: the labels might describe quarters or irregular observations.
Missing observations need an explicit treatment. A blank is not a zero. Joining the last observed point to the next available point may be a useful drawing convention, but it does not measure the missing interval. Write down where the gap is and avoid inventing a path through it.
The start date should fit the question. If you choose it only after seeing a dramatic rise, the resulting story may hide an earlier fall. A useful check is to look at a wider window and state whether the conclusion changes.
A worked example
A is £100 on 1 January and £120 on 1 April. B is £200 on 1 February and £230 on 1 April. The changes are 20% and 15%, but the start dates differ. You need A’s February value or B’s January value to compare changes over an identical interval.
Your turn
You have four observations labelled January, February, April and July. Can you describe them as four consecutive monthly values?
Open the exercise tool →Show the answer and reasoning+
No. March, May and June are missing. Keep the actual dates and show the gaps. “Four recorded observations between January and July” is accurate; “four consecutive months” is not.
Match dates before comparing changes.
Related term
Time windowSources & review
Original Charts Haven example. All stated numbers are illustrative and allow you to repeat the calculation.